The present invention relates generally to the field of display devices. More specifically, the present invention relates generally to dimming methods and apparatuses for lamps used in backlighting systems for display devices, such as liquid crystal display ("LCD") devices.
LCD devices are used widely in many applications, including, for example, aircraft instrument display systems. An LCD device includes a liquid crystal panel selectively made opaque in certain regions in order to generate images, icons, and characters in an instrument display in response to, for example, a video signal. To further enhance the visibility of such images of the liquid crystal panel, LCD devices require a backlight, i.e., a light source positioned on the backside of the liquid crystal panel. In recent years, LCDs with backlights have been incorporated into the cockpits of all types of aircraft. The aircraft cockpit can be one of the most extreme environments in which a fluorescent lamp must operate. As applied to aircraft instrument display systems, especially in military aircraft display systems, it is important that the LCD device have the functionality to dim the luminance of the LCD panel.
One aspect of the cockpit environment which affects the backlight system is the large dimming range. These LCDs require a backlighting system to make information visible to the pilot under lighting conditions that can range from near blackness at night to direct sunlight on the LCD during the day. As such, an LCD that operates in this environment must have an extremely-high dimming ratio. Because it is also desired that the backlighting color not change over the dimming range, fluorescent lamps are preferred because their color is not altered by dimming but rather by the selection of the appropriate composition of phosphorous coating within the lamps. Accordingly, the brightness of the fluorescent lamp needs to vary by large amount in order for the pilot to be able to view the LCD under all lighting conditions. The system should be free of swirls, flicker, and discontinuities and be capable of withstanding temperatures from--55.degree. C. to 85.degree. C. with a smooth response to the pilot's dimming command and be able to provide a large number of cold starts and hours of operation while maintaining a high-efficiency circuit.
One scheme for dimming a fluorescent lamp is a system in which the alternating signal that is supplying power to the lamp is cut with a notch of variable width so as to reduce the power applied to the lamp and thereby provide the desired dimming. The smaller the widths of AC power provided to the lamp, the lower the luminance at which the lamp operates. A common device for providing the ability to vary the width of the pulses are commercially-available pulse-width modulators ("PWM").
A PWM is a device that causes pulse-time modulation (modulation in which the value of instantaneous samples of the modulating wave are caused to modulate the time of occurrence of some characteristic of a pulse carrier) in which the value of each instantaneous sample of the modulating wave is caused to modulate the duration of a pulse. The modulating frequency can be fixed or variable. The basic operation of these PWMs is as follows. A reference voltage is transmitted to the PWM. The magnitude of the reference voltage is proportional to the desired width of the pulses.
The present invention is a dimming device that dims the fluorescent lamp of a backlight of an LCD device. The present invention provides a factor of ten improvement over conventional dimming devices without increasing the cost of such a dimming device by any significant amount.